The steering compass on any vessel remains essential to navigation.  It will almost certainly be a magnetic compass and as such will be subject to various errors that we must be aware of and take into account for safe navigation.

There are two main errors affecting magnetic compasses on any boat. Variation and Deviation.

VARIATION - The difference between True North and Magnetic North.  Variation for your local area can be found by looking at the Compass Rose on your chart, it shows a magnetic north arrow and shows the local variation and the annual rate of change.

 

 

 

 

DEVIATION - Ferrous metal (engine etc) and electrical equpment can deflect the compass from magnetic north.  A compass corrector can adjust your steering compass to minimise errors and then produce a Deviation Card showing the remaining deviation.  You can also 'swing' your own compass and produce your own deviation card.

 

 

 

 

 

DEVIATION CARD (Curve type) DOWNLOAD PDF

DEVIATION CARD (Tabular type) DOWNLOAD PDF

 

ALLOWING FOR VARIATION AND DEVIATION

To convert a course to steer True (T) to a course to steer Compass (C) we must allow for both Variation (V) and Deviation (D).  A simple way to remember the order of work is by using the mnemonic 'True Virgins Make Dull Company' and the word CADET (Compass to True Add East) to remind us if we are adding or subtracting.

 

 

If we are converting from Compass to True the word CADET reminds us that we ADD any Eastery variation and deviation, obviously any Westerly variation or deviation would be subtracted.

If we are coverting True to Compass the reverse is correct in that we would SUBTRACT any Eastery variation or deviation and add any Westerly variation or deviation.

 

 

Using this step by step method of working between True and Compass greatly reduces the chances of making mistakes.